IMO in the end everyone should lengthen the other segment too in order to regain natural biomechanics which is 0.8 tibia to femur ratio.
That's why it's better to not lengthen tibia more than 5.5 cm as you then have to lengthen 7 cm on your femurs to regain 0.8 tibia to femur ratio.
5.5 cm on tibia is already stretching it and can be done without permanent achilles tendon problems if one is really flexible and young enough.
Now, 5.5 cm tibia means that you have to lengthen exactly 7 cm on your femurs (no more and no less) which is considered the same in safety profile as 5.5 cm tibia lengthening.
So, basically, with double LL it's only safe to lengthen up to 5 inches exactly (more likely 12.3 cm if we subtract the Q angle from femur).
12.3 cm net height increase is a huge jump from any starting height IMO and is only worth it if one is really short like between 5'5-5'7. If someone is 5'9 for example, he must either do 5 cm tibia and be done with it or if he cares about perfect leg proportions and have enough money, time and energy should do 3.5 cm tibia and 4.5 cm femur for 3 inches total max as there's no point in risking more than that if one is hovering between 5'8-5'9.
IMO nobody should undergo this grueling procedure more than once if someone can get away with increasing tibia length up to 5 cm to stand at 5'10-5'11 and at 5'10 and above nobody should think about LL unless it's for deformity or obvious disproportion correction between limb segments (so, height gain should be a secondary goal in this case).
For anyone below 5'8 like guys 5'5-5'7 have no choice other than doing both segments as if one does 8 cm femurs he will look deformed until he does at least 4.5-5 cm tibia too.
So, unfortunately, no single LL for guys who need more than 5 cm as doing femur for just 5 cm is pointless as it's much expensive and doing tibia for more than 5 cm is asking for trouble and a matter of luck.
Anyway, if one lengthens more than 5.5 cm on tibia he will look disproportional nked until he lengthens femurs too for at least the same amount (ideally 1.25x amount of tibia) to regain proportions and biomechanics.
So, the best route of someone who needs double LL to become average height is to do tibia first and never go above 5.5 cm and then do the femurs for exactly 7 cm to regain perfect proportions between the limbs.
I think that the best aesthetic result I've ever seen on a 5'5 guy who did both segments is jim_dabarber who did 4 cm on tibia and 5 cm on femur for 3.5 inches of total height gain. The final photo he posted in his diary when he regained the muscles on his leg was very aesthetic and natural.
Unlike other dudes who post post lengthening photos with clothes he posted the pick nked with only underwear to show off his whole body proportons after the lengthening and rehab.
This is how he looks like and I think that 4 cm on tibia and 5 cm on femur is the ideal goal in terms of safety and proportions.
https://imgur.com/gallery/HyeZIFfBut, if someone has really short legs and big upper body and wants to gain the maximum safe height from double LL than 12.5 cm is the upper limit where if someone's starting arm length and shoulders width is fine enough to look acceptable post lengthening. Anything more than 2.3 inches on tibia and 3 inches on femur will look bad on any starting height (especially the lower the starting height is).
So, basically, 5'5 guys can become 5'10 maximum and look acceptable only if they have really short legs compared to upper body and at least the same wingspan as his original height + has decently broad shoulder and frame.
IMO jim_dabarber could pull of 5.5 cm tibia and 7 cm femurs and still look acceptable.